I have a moderate number of Sylvania clear No. 25 flashbulbs from various lots circa 1965 thru 1972. Most of the time, I calculate the f-stop and open up one stop and get great results with Ilford HP5 with the 4x5 Crown Graphic (Compur) and the Super Graphic (Graphex) at 1/125 or 1/250 M-sync.

Once in a while (perhaps one exposure in 8-10) there is significant underexposure where the image is too underexposed to be printable. I check the "blue dot" on the lamp before firing and they are indeed blue. I can almost tell by the "crackle" of the bulb that the shot will be underexposed. This happens with either one of my 4x5 Graphics or my Rolleicord so I've ruled out erratic M-Sync as the culprit.

I also lightly clean the tip connection of the lamp with fine emery paper and keep the contacts clean in the Graflite gun.

I use fresh Alkaline D-cells in the flash and have verified the sync cord connection with a 6-volt automotive lamp that will fit in the flash gun and glows when I shunt the PC or Bipost connection.

I use these to get a "vintage" look on occasional pictures.
I more often use my old Metz strobe to conserve these old bulbs....

I have 93 (count-'em-nine-three) no. 5 and 5B bulbs of various ages, the most recent being the 5B 12-pack I bought at the Kmart back in the '80s. I haven't had a chance to fire any yet, in my "new" Graflite Jr. flash, so it remains to be seen whether they are still good. Fortunately I also have a Kodak Master Photoguide from 1954 that has all the info one could need for calculating flash exposure. You do have to be careful to convert the ASA values for today's film, though, as these have changed through the years.